OK, I written a few posts arguing that Dick, Batman, and Joker exist in a trinity of morals: Dick represents ultimate good, Joker represents ultimate evil, and Batman operates in the middle as the righteous, flawed man. Other ComicViners have also related the other Robins to Batman thematically, so I'll give my two cents:
Jason Todd represents Batman's anger, fear, and resentment. Jason was the violent Robin who acted on his impulses and put himself above his prey - humans deserving of life. He was also the Robin who feared for his place in the world, the way Batman sought for identity in his formative years. Jason was Robin for four years post-Crisis, maybe a year in the comics before he died, but Batman internalized his essence so strongly that afterwards he nearly turned into the murderer of his origins; in a way, he never left.
Tim Drake represents Batman's logic, calculation. Tim was an equalizer, Dick approved, who helped restore balance to Batman. But as Tim got older, readers gained insight into a colder, clinical version of Bruce, a version where the finesse was replaced by an efficiency that could even factor homicide into a neat little strategy.